I suspect there are a lot of 'Centenary' models out there that don't wear the 'one in a thousand script' on the air cylinder, until recently, it was possible to purchase 'New /old stock' - Centenary stocks that would make it a fairly simple operation to convert an 'S' model, of course the serial number would be wrong, but essentially it would be the same to shoot. Nothing wrong with that mind, I don't personally have any problem with that unless someone was trying to sell it/pass it off as one.
If I ever get around to doing it (I have the hardwood blank ready) My project Mk1 will wear a handmade/one off stock that will broadly follow the elongated format of the Centenary stock but it will feature a higher comb/cheekpiece for the scope it will wear as it will feature a scope ramp fitted on top of the air chamber.

(and for the 'purists' out there) The Mk1 I intend to modify thus - I acquired specifically as it was in good working but poor overall cosmetic condition (and minus a stock) and needs a complete overhaul and will be getting a lot of money/attention and time lavished on it! (and it will be loved)

Don't think I will be getting 'One of only one ever' engraved on it though!

I like the 'Centenary' rifle - just a pity they didn't base it on the Mk1 action instead of the 'S' - now that would have been something. (just my opinion )